Precession of the Equinoxes, page 2
The earth's axis is tilted at 23.5° from the vertical (that is, compared to its orbit around the sun), and this produces some noticeable effects, chiefly the seasons and their varying hours of daylight. In the Northern Hemisphere:
Between the two extremes there is a day (one in spring, and one in autumn) where day and night are equal in length, that is 12 hours each. Each of these days is called an equinox:
In the days of yore, when we had no calendars, marking the passage of the seasons was vitally important, and these four days provided a marker every three months, if only we could spot them.
Ancient farmers couldn't fail to notice that the sun rose or set at progressively different places along the eastern and western horizons during the year. It would not have taken them long to figure this was a regular occurrence and could be used to mark the passage of the year. For farmers, marking the equinoxes was vital because:
It is probable that two rules evolved amongst those who survived starvation:
It's not difficult to mark the equinoxes as the sun rises and sets at each end of a straight line, due east and due west. A sighting line drawn in the ground is all you'd need, or two posts, or two stones, to mark these special days. Our ancient farmers survived, and we are here today precisely because of their cleverness.